AR5: cursory review of chapter 4 (cryosphere) mass balance of Antarctica

The AR5 draft is now available from W00t! In the style we've come to love so much from the IPCC it sayeth:

The final draft Report, dated 7 June 2013, of the Working Group I contribution to the IPCC 5th Assessment Report "Climate Change 2013: The Physical Science Basis" was accepted but not approved in detail by the 12th Session of Working Group I and the 36th Session of the IPCC on 26 September 2013 in Stockholm, Sweden. It consists of the full scientific and technical assessment undertaken by Working Group I. The Report has to be read in conjunction with the document entitled “Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the IPCC 5th Assessment Report - Changes to the underlying Scientific/Technical Assessment” to ensure consistency with the approved Summary for Policymakers (IPCC-XXVI/Doc.4) and presented to the Panel at its 36th Session. This document lists the changes necessary to ensure consistency between the full Report and the Summary for Policymakers, which was approved line-by-line by Working Group I and accepted by the Panel at the above mentioned Sessions.

(My bold). So that's a pain up the f*ck*ng arse then - anything I want to read I have to first check they haven't revised? Spare me. Note, though, that the revisions seem to be of the sort Trickleback Figure 4.4 The trends numbers need to be removed to avoid the excessive significant figures, and ensure compatibility with numbers provided for SPM, which are given for different likelihood statement. The uncertainties are likely not very likely, so is confusing! Joey Comiso to provide revised figure, with trends given to match numbers in the SPM - i.e. the tedious process of aligning stuff, so I doubt I have to care very much.

Oh, but before I leave off insulting them: what kind of idiot puts up a document of such obvious public interest and then writes on it "Do Not Cite, Quote or Distribute"? Will someone please fire the IPCC press office and get them someone competent.

I decided to read Chapter 4 (obs; cryosphere) and specifically the Antarctic stuff. And specifically the mass balance of Antarctica, since I used to know about that a bit. So I'm looking for the text supporting:

The Antarctic Ice Sheet has been losing ice during the last two decades (high confidence). There is very high confidence that these losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, and high confidence that they result from the acceleration of outlet glaciers. [4.4.2, 4.4.3, Figures 4.14, 4.16, 4.17]

That's interesting to me, because when I started (and arguably when I stopped, around 2007) the overall mass balance of Antarctica was unknown, including its sign. That's because in general you were trying to difference two large numbers (accumulation at the top, against loss form the edges) and the uncertainties in either were bigger than the difference. For comparison, AR4 said:

Taken together, the ice sheets in Greenland and Antarctica have very likely been contributing to sea level rise over 1993 to 2003... Estimates for the overall mass balance of the Antarctic Ice Sheet range from +100 to –200 Gt yr–1 (–0.28 to 0.55 mm yr–1 SLE) for 1961 to 2003, and from +50 to –200 Gt yr–1 (–0.14 to 0.55 mm yr–1 SLE) for 1993 to 2003. The recent changes in ice flow are likely to be sufficient to explain much or all of the estimated antarctic mass imbalance...

So what has changed? Well page 4-28 says:

Since AR4, satellite, airborne and in situ observations have greatly improved our ability to identify and quantify change in the vast polar ice sheets of Antarctica and Greenland. As a direct consequence, our understanding of the underlying drivers of ice-sheet change is also much improved

Hmm, OK. But in detail? There are three methods: mass balance, altimetry, and GRACE (I last wrote about GRACE in 2009 I think). "The mass budget method (see Glossary) relies on estimating the difference between net surface balance over the ice sheet (input) and perimeter ice discharge flux (output)". They discuss the net sfc mass balance, and provide numbers. Oddly, they don't provide any numbers for the discharge side. Also, if you look at Table 4.A.3 "Sources used for calculation of ice loss from Antarctica" you'll find that they've used GRACE overwhelmingly.Aside from which there are two others: they also used "Shi et al., 2011" but as they say "Methodology and error budget incompletely described" and also it arrives at an uncertainty of 5 Gt/yr, which is implausibly precise, so forget that. And they used Rignot et al. 2011c. That uses the std-ish long trail of stuff (ice velocities, corrections for grounding line movement, etc. etc.) to get you as far as ice discharge rates. And that leaves you with an overall value that makes the mass balance negative. I'd be slightly iffy about whether the values are really known closely enough, but the point (in Rignot too, since they use it) is that it agree with GRACE, so So There we've got close enough and can stop now.

But overall, it looks pretty clear that the main advance has come from using GRACE. Which is really funky and fairly direct, so good. Note, BTW, that although I've been somewhat sniffy in my I-used-to-know-this-stuff way about the absolute value of the contribution from Antarctica, this isn't desperately important - what matters is the change over time, and everyone agrees (now, unlike AR4) that this is leading to SLR.


* Victor Venema at "Variable Variability" is collecting other (generally more temperate) reviews of AR5.
* A Reconciled Estimate of Ice-Sheet Mass Balance

More like this

Why release the document before it's finished? We've waited seven years, we can wait a bit longer.

By andrew adams (not verified) on 01 Oct 2013 #permalink

Andrew Adams, publishing the "draft" now makes it possible to verify that the Summary for Policy Makers that was published last Friday corresponds to the full report.

By Victor Venema (not verified) on 01 Oct 2013 #permalink

Victor, but where they do not correspond, that just means the full report is going to be changed :-)

[Ah, Mialambre, welcome. We await your views on the description of Climate Sensitivity in AR5 with interest -W]

By James Annan (not verified) on 01 Oct 2013 #permalink

:-) Yes, but then, I do not fear change.

The unreasonable will complain when things changed because they changed and they will complain when things stayed the same because they stayed the same.

By Victor Venema (not verified) on 02 Oct 2013 #permalink

> fire the IPCC press office

I read that the IPCC has only a dozen people on the paid staff, so if you fire the press office, you may also be firing the janitor and the mail room.

By Hank Roberts (not verified) on 02 Oct 2013 #permalink

> We await your views on the description of Climate Sensitivity in AR5 with interest -W

'Bated breath', actually, from one more member of the fan club

By Martin Vermeer (not verified) on 03 Oct 2013 #permalink

Found this, William, would appreciate your thoughts - related though not about climate sensitivity as such:
thanks, fergus

[You have at least one thing subtly wrong - you paraphrase them as "shrinking faster" but they don't actually say that in the abstract you quote. Its not incompatible with their "most glaciers and ice caps are farther from balance than previously believed" but its not quite the same thing (they could simply be implying that more mass loss will occur long-term).

On the point of estimates of SLR, I've come to be sympathetic to the idea that the semi-empirical models - like these, like (I think) Rahmstorf's stuff - may be a better (and higher) estimate for the coming century than the GCMs. As an ex-GCMer that's not a nice thing to say. The GCMs are able to wrap up a lot of future-prediction stuff, but they lack detail; and the semi-empirical stuff just handles that detail (although at the risk of doing it wrongly) -W]

By Fergus Brown (not verified) on 04 Oct 2013 #permalink

The paper is embedded behind the title. Which I read. So, what's the distinction between 'further from mass balance' and 'shrinking'? The paper seems to be suggesting the latter, and since I'm writing as a simpleton, I thought my paraphrase was fair, though pleased to be corrected...

By Fergus Brown (not verified) on 04 Oct 2013 #permalink

“Do Not Cite, Quote or Distribute” -- presumably the AR5 form of a disavowal statement, so they can say whatever they want whilst denying any accountability. They are counting on the prostrate media to perpetrate the alarmism while they themselves, the font of it, can deny as needed.


Looking at the AR5 table for projected glacier SLR contribution estimates (Table 13.3, page 28 chapter 13), they do list that paper's predecessor which gives a fairly similar result so this wouldn't be anything new for the AR5 authors.

I think the issue is that their estimate relates to the state of glaciers after adjusting/equilibrating to current climate conditions, but they provide no timescale for that adjustment. Is it going to take a decade or a millenium?

The long-term process projections in chapter 13 (figure 13.13, table 13.8) indicate significant continued glacier contributions past 2100, even under lower (<500ppm) CO2 scenarios. There are various scenarios under this umbrella but typically they force a relatively small GAT increase of ~0.6ºC by mid-21st Century then stabilisation or slight decline through to 2500. There is no temperature increase beyond 2050 yet glaciers continue to decline up to 2300 when they contribute ~ 24cm SLR compared to ~ 16cm by 2100. This indicates a full adjustment timescale of about 200 years in process models.


My issue with the paper (not referred to in the blog entry) is the fact that they indicate an imbalance based on current conditions, without factoring in GAT changes. Which makes me wonder for the future is the likelihood that the imbalance will increase, in the real world. Unless I have missed something...

By Fergus Brown (not verified) on 08 Oct 2013 #permalink


We may be talking at cross-purposes. Certainly any imbalance would increase should GAT increase. My point in referencing results from low-CO2 scenario modelling was that GAT stabilises around 2050, and isn't a long way from current climate conditions, but glaciers continue to significantly lose mass up to 2300. The implication is that the presence of a large imbalance wouldn't necessarily be something that significantly affects estimates of SLR up to 2100, which is mostly what AR5 reports.

Anyway, glacier estimates are probably not worth losing sleep over. Process models indicate ~15cm SLR by 2100, empirical models ~25cm. Even the absolute worst case scenario where glaciers completely disappear by 2100 means only a 45cm SLR. In terms of real world implications ice sheet contributions should be the focus.

Moving to a thread-relevant tangent, Ice sheets are where I have a problem with the GAT-calibrated semi-empirical SLR estimates. SLR of 1m to 1.5m (and above) implicitly assumes a considerable ice sheet contribution, and that seems to be the main point of difference between semi-empirical and process estimates. However, semi-empirical methods provide no direct evidence to support that larger contribution, so I don't see what they add to the discussion.

As you say, Paul, getting back to the relevant stuff. After William's comments I hooked in and dug. Principally via TC and TCD, most recent recalculation of ice sheet mass balance seems to be pushing the numbers further into the negative, whether through more detailed modelling or simply more available information. As study improves understanding, those semi-empirical approaches (we had this before - there's nothing to replace being there and seeing it) look increasingly credible in terms of their projections, and the AR5 numbers steadily look increasingly conservative. Though I would point out that my confidence that SLR will exceed 1 m by 2100 is currently no better than 'medium'. :)

By Fergus Brown (not verified) on 09 Oct 2013 #permalink

Oh, I'm not at all suggesting that process models are getting things right :) Stefan made a good point at RealClimate a few months ago along the lines of: semi-empirical estimates may have issues, but it's not clear why we should believe process-based estimates are any better.

The essence of my point is that SLR over the past hundred years is not considered to have been influenced much at all by ice sheet mass balance, except in the last decade or so. When GAT change is calibrated with SLR in these semi-empirical methods that means very little of the input has anything to do with ice sheets but the output projections disproportionately depend on ice sheet contributions due to the total source size of other contributors (i.e. glaciers).

Therefore, since ice sheets are the main issue and semi-empirical estimates don't have much to say about them as yet, I don't see how they can be trusted to provide useful information about the future. They might turn out to be about right, but it's not clear why they should be ;)

The estimate from that paper looks to be a close match to the AR5 2005-2010 estimate of about 1mm/yr sea-level equivalent from Antarctic+Greenland ice sheets. As I understand it though, the issue is whether this is a trend which will accelerate, stabilise or slow down and most investigations into the details have suggested that significant acceleration is unlikely to continue over the course of the next century (e.g.… and…).

> what matters is the change over time

(including the change in what we understand about the rate of change over time). My favorite example:
illuminates how fast our ideas about physical processes change, and how long our perplexity persists even as we learn more and more faster and faster.

Slow melting, or catastrophic outburst erosion?
Time will tell.

By Hank Roberts (not verified) on 11 Oct 2013 #permalink

(brief quote from that last one as it's very slow to load)
"Although erosion of valleys by subglacial meltwater was first proposed more than 100 years ago, the processes that produced these valleys under past ice sheets have remained a topic of vigorous debate. ...."
Earth-Science Reviews Volume 113, Issues 1–2, June 2012, Pages 33–58

By Hank Roberts (not verified) on 11 Oct 2013 #permalink